Pig iron process



Oct. 11, 1966 ss ETAL 3,278,299

PIG IRON PROCESS Filed Aug. 20, 1962 INVENTOR HARRY H. KESSLER WILLIAM H. MOORE Mflm v XI TORNEYS United States Patent 3,278,299 PIG IRON PROCESS Harry ll-I. Kessler, 7 Dromara Road, Clayton, Mo., and William H. Moore, 19 Villa Road, Larchmont, N.Y. Filed Aug. 20, 1962, Ser. No. 218,074 2 Claims. (Cl. 75130) This invention relates to an improved method of making foundry pig iron for remelting purposes and the product thereof. Foundry pig iron, as normally constituted, contains carbon in the form of graphite or free carbide, with the majority of all pig iron being of the graphitic type. This graphite consists of large spiky or hornlike particles of kish or primary graphite, together with large secondary flake graphite of the saucer variety. This is called type C graphite according to the ASTM-AFS graphite classification chart.

An object of this invention is to provide a pig iron having the graphite in an improved compacted or nodular form, with substantially reduced quantities of spiky kish. Another object is to provide a pig iron highly purified with respect to elements that promote flake graphite. Another object is to provide a pig iron for remelting purposes which, when remelted, will produce a melt of improved physical and mechanical property potential.

Still further objects will become apparent on the reading of the specifications and drawings in which:

FIGURE 1 is a photomicrograph taken at 100 diametrs showing the typical spiky kish and saucer flake graphite of a commercial pig iron;

FIGURE 2 is a photomicrograph taken at 100 diameters showing the nodular and compacted flake graphite of a pig iron made according to the teaching of this invention;

FIGURE 3 is a photomicrograph taken at 100 diameters showing the graphite obtained on remelting a typical commercial pig iron. Note the areas of kish graphite. AFS type C; and

FIGURE 4 is a photomicrograph taken at 100 diameters showing the graphite obtained on remelting the pig iron of this invention. Note realtive absence of kish graphite and improved distribution of flake graphite. AFS type A.

In the manufacture of cast iron castings, foundry pig iron has always formed a large proportion of the furnace charge. According to the economic position with regards to the relative cost of pig iron and cast iron scrap and steel, the proportion of pig iron in the furnace charge may be varied over fairly broad limits. However, in most types of cast iron used for various purposes, a certain minimum amount of pig iron is always preferred in the charge, as it is of known chemical analysis and confers certain useful foundry characteristics to the molten metal prepared from the furnace charge. Proportions of pig iron used in the average furnace charge may range from as little as to as high as 100% by weight, but is generally in the region of 25% to 40%.

It has been realized by those skilled in the art, that the type of pig iron used has a great influence on the resultant cast iron. This has often been referred to as heredity and has caused extravagant claims to be made regarding the use of certain types of pig iron prepared by special methods such as charcoal smelting or containing small quantities of special trace elements such as vanadium and titanium.

Regardless of the authenticity of various claims, it has been established beyond a doubt that the nature of the pig iron does have a measurable influence on the metallurgical and foundry character of the molten metal produced from a charge containing this pig iron.

In recent years, particularly since the advent of nodular cast iron and since it has become recognized that small quantities of special elements can exert a great effect on the nature of the graphite in cast iron, metallurgists have increased their efforts to obtain pig irons which are low in trace elements which might upset the metallurgy of their end products.

It has also been recognized by some, that the size, form, and shape of the graphite in the pig iron exerts an influence on the size, form, and shape of the graphite in a cast iron made from pig iron, or from a charge containing a proportion of pig iron.

In general, a pig iron containing large quantities of coarse flake graphite and large quantities of spiky kish will tend to produce a cast iron containing a similar graphite, whereas pig irons containing quantities of smaller flake graphite and less kish tend to produce cast irons themselves having smaller and improved graphite flakes.

There doesnt seem to be a great deal of difference between one pig iron and the next, except, perhaps, in the proportion of free carbide that exists versus the amount of graphite that exists-thus, a pig iron may be mottled or partially white and this pig iron, when melted in a charge, will usually produce a cast iron having a somewhat improved character of graphite, depending on the chemistry attained and the methods of melting and processing the melt.

As pig iron is usually added also to introduce silicon into the melt, it is much more normal to use pig irons of sufiiciently high silicon content that they are usually completely gray. When once a pig iron is completely gray, there is not too much choice available for the metallurgist who wants to improve the graphite structure of the cast iron produced from a charge containing pig iron.

It is recognized by those skilled in the art, that the melting process may not completely destry all the graphite flakes that exist in the pig iron used in the charge. For this reason, pig iron with coarse graphite will produce an iron of lower chill value than a pig iron of fine graphite. This is because the graphite is more difficult to get completely in solution and particles of graphite which are undissolved act as nuclei for the precipitation of further graphite from the melt.

We have discovered that pig iron containing the nodular, or partially nodular, form of graphite with little or no kish or primary graphite of the spiky variety, may be remelted to produce cast irons having considerably improved mechanical properties.

We have discovered that pig iron containing the nodular or partially nodular form of graphite with little or no kish or primary graphite of the spiky variety, may be used in malleable iron charges according to conventional practice and the result of using this pig iron is to produce a malleable iron having less tendency to primary graphite in heavy sections. It is also noticed that malleable iron made with the pig iron of this invention tends to malleablize more readily than with other types of pig iron conventionally used.

We have discovered also, that the use of these pig irons in producing cast irons having a nodular or spheroidal form of graphite, enables a melt to be made which may be nodularized considerably more readily with the use of less nodularizing agent than when conventional pig irons are used.

We have discovered also, that the use of pig iron containing graphite already in the nodular form gives substantially no difficulty in the manufacture of nodular iron with deleterious elements which may prevent the successful nodularization of the melt. The reason for the improvement in a cast iron produced with our improved pig iron is not known exactly, but it is presumed that it is tied in with the improved ability of the graphite in the pig iron to go completely into solution during melting and also, perhaps, with the presence of a carbide metastabilizing elfect in the pig iron left from the nodularizing procedure used in producing the pig iron.

Further than this, it is probable that when a pig iron has already been nodularized that all elements which would poison a cast iron melt with regards to nodularization have been successfully overcome and their effect on a subsequent melt produced with this pig iron would be negligible.

The exact method of producing the pig iron of our invention is not too important; thus, it may be produced in a blast furnace, in any other type of shaft furnace, or even in an electric furnace by reduction of iron ore and carbon, or by any of the conventional means.

In producing this pig iron the molten pig iron is subjected to a treatment which will nodularize the graphite in the melt. This treatment may utilize calcium, magnesium, cerium, lithium, or any of the other well known nodularizing agents and they may be added by sub-surface injection with inert gas, by direct surface treatment, or by any of the means known to those skilled in the art.

Wep-refer to use those elements such as the rare earths which, in themselves have been shown to be particularly beneficial in overcoming the subversive effects of certain tramp elements which may result in the pig iron from the ores used in its manufacture. For this reason, we may use small quantities of misch metal or we may use rare earth fluorides mixed with a reducing agent such as calcium silicide.

As an example of the method of producing the pig iron of our invention a portion of molten metal from a blast furnace was cast into a test piece and pigs marked A and to a further portion an addition was made comprising by weight of a mixture containing 80 parts of calcium silicide and 20 parts of rare earth fluoride. This metal portion was cast into a test piece and pigs marked B. To a third portion of the molten metal was added A by weight of a mixture consisting of 40 parts of calcium silicide, 40 parts of magnesium silicon alloy and 20 parts of calcium fluoride. This metal portion was cast into a test piece and pigs marked C.

The test pieces thus cast were examined along with the pigs cast with each portion and the results were as follows:

The pigs cast with each portion were used in commercial heats and found to give the benefits hereinafter demonstrated.

Regardless of the method of producing pig iron with the nodular form of graphite, we have discovered that such a pig iron, when used for the purposes of remelting and producing either flake graphite or nodular graphite cast iron can confer certain benefits to the melt which have not hitherto been realized. The essential feature of this invention may best be appreciated by a series of examples.

Example N0. 1.A melt was produced in a cupola from a charge consisting of 50% structural steel pieces and 50% of a regular foundry pig iron having the composition:

0.5 of silicon as lump ferro-silicon was also added to the charge. A portion of this melt was cast into 1.2" test bars and these were then tested with the following results:

Total carbon, percent 3.35 Silicon, percent 1.72 Manganese, percent 0.70 Tensile strength, p.s.i 33,000

A second melt was produced in the same furnace from a charge consisting of 50% structural steel pieces and 50% of the pig iron of this invention having the graphite in nodular form produced by adding a magnesium ferro silicon as a nodularizing agent of a first effective amount to a blast furnace pig iron and the composition:

Percent Total carbon 4.35 Silicon 2.52 Manganese 0.85

0.5% silicon, as lump ferro-silicon was also added to the charge. A portion of this melt was cast in 1.2 test bars and these were tested with the following results:

Total carbon, percent 3.41 Silicon, percent 1.69 Manganese, percent 0.73 Tensile strength, p.s.i 42,000

An examination of the microstructure of these test bars showed a very much smaller and improved graphite flake in the heat that had been produced with the pig iron of this invention. This was reflected by the substantial increase in tensile strength shown above.

Example N0. 2.A melt was produced in an electric furnace using a charge consisting of 100% pig iron of composition:

Percent Total carbon 4.25 Silicon 1.30 Manganese 0.35 Sulphur 0.015

and having all its graphite present as spiky kish and saucer flake form. To this melt was added 1 /2 of magnesium ferro-silicon containing 9.5% magnesium and 40% of silicon. The treated melt was cast into test bars which were examined and tested. These bars showed fifty percent of the graphite in the nodular form with the balance as compacted flake graphite.

The chemistry was:

A second melt was produced in the same electric furnace using a charge consisting of 100% of the pig iron of this invention. This pig iron had previously been produced by adding by weight (first effective amount) of an to 20 mixture of calcium silicide and rare earth fluoride to molten iron from the blast furnace and casting the pigs. The pig iron had about 50% of the graphite in the nodular form with the balance as compacted flake graphite. The chemistry of the pig iron was:

Percent Total carbon 4.60 Silicon 1.38 Manganese 0.28 Sulphur 0.018

To the melt produced from this pig iron was then added 1 /2% of magnesium ferro silicon containing 9.5

magnesium and 40% of silicon with this amount constituting a second eifective amount. The second eifective amount of nodularizing agent being less than would have been used if the pig iron had been comprised substantially of type C AFS graphite and had not been nodularized by the first effective amount of nodularizing agent. This treated melt was cast into test bars which were examined and tested. These bars showed more than 90% of the graphite in the nodular form and the chemistry was:

Percent Total carbon 3.95 Silicon 2.65 Manganese 0.29 Sulphur 0.011 Magnesium 0.028

The physical properties were:

Tensile, p.s.i 8 2,000 Yield, p.s.i 48,000 Elongation, percent 17.0

This example clear-1y illustrates that all factors being equal, the pig iron which had previously been nodularized forms a melt which is easier to subsequently nodularize than a similar melt produced with conventional pig iron of the same composition.

The method of producing a nodular cast iron casting according to the teachings of the present invention and as demonstrated in the example may be said to comprise the steps of selecting an initial liquid iron, which if cast without treatment would comprise essentially type C AFS graphite characterized by spiky Lkish, and then adding an effective rfirst amount of nodularizing agent to the initial liquid iron and pouring and cooling the same to produce pig iron having at least some of the graphite in the nodular form and substantially no spiky kish. This pig iron is then added to a furnace charge and makes up from 110% to 100% of the charge, which is then melted and an effective second amount of nodularizing agent is added thereto. The effective second amount is less than would have been used it" the pig iron had comprised substantially type C AFS graphite and had not been nodularized by the effective first amount of nodularizing agent. The final step is the casting of the melt into a casting with the graphite being in the nodular form. Also in accordance with the teachings of the present invention and as exemplified by one of the examples, the charge using the pig iron prepared in accordance with the teachings of the invention may be melted and if cast would contain type A AFS graphite in improved amount over that which would have been obtained had the charge comprised essentially the initial liquid iron and been cast without the addition of the first effective amount of nodularizing agent. In this procedure the tfinal step is the casting of the melt into a casting with the graphite being a flake form.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that many variations may be resorted to in practice that will still come within the scope of this invention.

What is claimed is:

1. In the process of producing a nodular cast iron from a charge containing from 10% to of pig iron, that improvement comprising incorporating into the charge a nodular pig iron characterized by the substantial absence of primary 'kish graphite known as type C AFS graphite and by the presence of nodular graphite produced by nodular treatment of the pig iron at the blast furnace with a nodularizing agent, melting said charge which includes said nodular pig iron to produce a molten bath, said nodular pig iron constituting from 10% to 100% of said charge and acting in the direction of improving the nodularizing tendency of said bath, treating said bath with a nodularizing agent to produce a nodular iron casting of improved nodularity at a lower residual nodularizing agent content than would have been the case if said nodular pig iron had not been incorporated in said charge, and thereafter pouring said molten bath into a casting With the graphite thereof being in the nodular form.

2. In the process of producing a cast iron with the graphite in the flake form from a change containing from 10% to 100% of pig iron, that improvement comprising incorporating into the charge a nodular pig iron characterized by the substantial absence of primary kish graphite knownas type C AFS graphite and by the presence of nodular graphite produced by nodular treatment of the pig iron at the blast furnace with a nodularizing agent, melting said charge which includes said nodular pig iron .to produce a molten bath, said nodular pig iron constituting from 10% to 100% of said charge and acting in the direction of improving the nodularizing tendency of the bath, and thereafter pouring said molten bath into a casting with the graphite thereof being in the flake form.

References Cited by the Examiner FOREIGN PATENTS 228,442 5/ 1958 Australia.

DAVID L. R'ECK, Primary Examiner.

H. TARRING, Assistant Examiner. 

1. IN THE PROCESS OF PRODUCING A NUDULAR CAST IRON FROM A CHARGE CONAINING FROM 10% TO 100% OF PIG IRON, THAT IMPROVEMENT COMPRISING INCORPORATING INTO THE CHARGE A NODULAR PIG IRON CHARACTERIZED BY THE SUBSTANTIAL ABSENCE OF PRIMARY KISH GRAPHITE KNOWN AS TYPE C AFS GRAPHITE AND BY THE PRESENCE OF NODULAR GRAPHITE PRODUCED BY NODULAR TREATMENT OF THE PIG IRON AT THE BLAST FURNACE WITH A NODULARIZING AGENT, MELTING SAID CHARGE WHICH INCLUDES SAID NODULAR PIG IRON TO PRODUCE A MOLTEN BATH, SAID NODULAR PIG IRON CONSTITUTING FROM 10% TO 100% OF SAID CHARGE AND ACTING IN THE DIRECTION OF IMPROVING THE NODULARIZING TENDENCY OF SAID BATH, TREATING SAID BATH WITH A NODULARIZING AGENT TO PRODUCE A NODULAR IRON CASTING OF IMPROVED NODULATITY AT A LOWER RESIDUAL NODULARIZING AGENT CONTENT THAN WOULD HAVE BEEN THE CASE IF SAID NODULAR PIG IRON AND NOT BEEN INCORPORATED IN SAID CHARGE, AND THEREAFTER POURING SAID MOLTEN BATH INTO A CASTING WITH THE GRAPHITE THEREOF BEING IN THE NODULAR FORM. 